1. Field of the Invention
The present invention relates to a motor for use in an apparatus, such as an image forming apparatus, which uses a motor (in particular, a vibration motor) as a drive source thereof.
2. Description of Related Art
Motors used, e.g., as drive motors for apparatuses of various kinds include DC motors, stepping motors, AC motors, vibration motors, etc. Among these motors, a vibration motor is desirably used for driving a photosensitive drum or the like of a copying machine (image forming apparatus). The vibration motor is composed of a vibrating body, which is arranged to be excited to vibrate by high-frequency voltage signals (a driving signal) applied to electric-to-mechanical energy converting elements (electromechanical energy conversion elements), and a contact body, which is arranged to be in contact with the vibrating body, as disclosed, for example, in Japanese Laid-Open Patent Applications No. SHO 58-14682, No. SHO 63-1379, No. SHO 60-176470 and No. SHO 59-204477. The vibrating body and the contact body are thus arranged to be frictionally driven relative to each other by the vibration energy of the vibrating body.
Since the vibration motor excels in speed stability at a low speed and thus does not require speed reduction by means of gears or the like, operations can be performed at a low noise level. In a copying machine, which is required to operate at low levels of vibration and noise, therefore, the use of a vibration motor is most suited for accurate control of the rotation of a photosensitive drum, which directly influences the picture quality of a copy.
The vibration motor, however, has presented a problem in the following point. In the event of dew condensation within the vibration motor as a result of a sudden change in ambient temperature, the moisture of the dew might cause the vibrating body and the contact body to stick to each other. To relieve the vibration motor from such a sticking state, it has been practiced to automatically raise the high-frequency voltage signal by a feedback control means, which is arranged to compare the above-stated applied high-frequency voltage signal with the driving state of the vibration motor and to control the high-frequency voltage signal according to the result of comparison.
However, the sticking state of the vibration motor sometimes cannot be readily removed by simple raising the high-frequency voltage signal, and thus brings about an inadequate building-up state, depending on the degree of sticking. Under such a condition, the photosensitive drum driven by the vibration motor cannot be synchronized with a transfer belt which is driven by another drive source. The failure of synchronization tends to cause the transfer belt to be damaged.
Further, continuous application of the high-frequency voltage signal causes the vibration motor to overheat, and thus might cause some trouble.
Further, repeating an attempt to forcibly drive the vibration motor in a sticking state degrades the wow flutter of the vibration motor, thereby making it hardly possible to obtain a stable motor rotation.